# std::sqrt(std::valarray)

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< cpp‎ | numeric‎ | valarray

Defined in header `<valarray>` |  |
template< class T > valarray<T> sqrt( const valarray<T>& va ); |  |

For each element in va computes the square root of the value of the element.

### Parameters

va | - | value array to apply the operation to

### Return value

Value array containing square roots of the values in va.

### Notes

Unqualified function (sqrt) is used to perform the computation. If such function is not available, std::sqrt is used due to argument-dependent lookup.

The function can be implemented with the return type different from std::valarray. In this case, the replacement type has the following properties:

  - All const member functions of std::valarray are provided.
  - std::valarray, std::slice_array, std::gslice_array, std::mask_array and std::indirect_array can be constructed from the replacement type.
  - For every function taking a const std::valarray<T>& except `begin()` and `end()`(since C++11), identical functions taking the replacement types shall be added;
  - For every function taking two const std::valarray<T>& arguments, identical functions taking every combination of const std::valarray<T>& and replacement types shall be added.
  - The return type does not add more than two levels of template nesting over the most deeply-nested argument type.

### Possible implementation

    template<class T>
    valarray<T> sqrt(const valarray<T>& va)
    {
        valarray<T> other = va;
        for (T& i : other)
            i = sqrt(i);

        return other; // proxy object may be returned
    }

### Example

Finds all three roots (two of which can be complex conjugates) of several Cubic equations at once.

Run this code

    #include <cassert>
    #include <complex>
    #include <cstddef>
    #include <iostream>
    #include <numbers>
    #include <valarray>

    using CD = std::complex<double>;
    using VA = std::valarray<CD>;

    // return all n complex roots out of a given complex number x
    VA root(CD x, unsigned n)
    {
        const double mag = std::pow(std::abs(x), 1.0 / n);
        const double step = 2.0 * std::numbers::pi / n;
        double phase = std::arg(x) / n;
        VA v(n);
        for (std::size_t i{}; i != n; ++i, phase += step)
            v[i] = std::polar(mag, phase);
        return v;
    }

    // return n complex roots of each element in v; in the output valarray first
    // goes the sequence of all n roots of v[0], then all n roots of v[1], etc.
    VA root(VA v, unsigned n)
    {
        VA o(v.size() * n);
        VA t(n);
        for (std::size_t i = 0; i != v.size(); ++i)
        {
            t = root(v[i], n);
            for (unsigned j = 0; j != n; ++j)
                o[n * i + j] = t[j];
        }
        return o;
    }

    // floating-point numbers comparator that tolerates given rounding error
    inline bool is_equ(CD x, CD y, double tolerance = 0.000'000'001)
    {
        return std::abs(std::abs(x) - std::abs(y)) < tolerance;
    }

    int main()
    {
        // input coefficients for polynomial x³ + p·x + q
        const VA p{1, 2, 3, 4, 5, 6, 7, 8};
        const VA q{1, 2, 3, 4, 5, 6, 7, 8};

        // the solver
        const VA d = std::sqrt(std::pow(q / 2, 2) + std::pow(p / 3, 3));
        const VA u = root(-q / 2 + d, 3);
        const VA n = root(-q / 2 - d, 3);

        // allocate memory for roots: 3 * number of input cubic polynomials
        VA x[3];
        for (std::size_t t = 0; t != 3; ++t)
            x[t].resize(p.size());

        auto is_proper_root = [](CD a, CD b, CD p) { return is_equ(a * b + p / 3.0, 0.0); };

        // sieve out 6 out of 9 generated roots, leaving only 3 proper roots (per polynomial)
        for (std::size_t i = 0; i != p.size(); ++i)
            for (std::size_t j = 0, r = 0; j != 3; ++j)
                for (std::size_t k = 0; k != 3; ++k)
                    if (is_proper_root(u[3 * i + j], n[3 * i + k], p[i]))
                        x[r++][i] = u[3 * i + j] + n[3 * i + k];

        std::cout << "Depressed cubic equation:   Root 1: \t\t Root 2: \t\t Root 3:\n";
        for (std::size_t i = 0; i != p.size(); ++i)
        {
            std::cout << "x³ + " << p[i] << "·x + " << q[i] << " = 0  "
                      << std::fixed << x[0][i] << "  " << x[1][i] << "  " << x[2][i]
                      << std::defaultfloat << '\n';

            assert(is_equ(std::pow(x[0][i], 3) + x[0][i] * p[i] + q[i], 0.0));
            assert(is_equ(std::pow(x[1][i], 3) + x[1][i] * p[i] + q[i], 0.0));
            assert(is_equ(std::pow(x[2][i], 3) + x[2][i] * p[i] + q[i], 0.0));
        }
    }

Output:

    Depressed cubic equation:   Root 1:              Root 2:                 Root 3:
    x³ + (1,0)·x + (1,0) = 0  (-0.682328,0.000000)  (0.341164,1.161541)  (0.341164,-1.161541)
    x³ + (2,0)·x + (2,0) = 0  (-0.770917,0.000000)  (0.385458,1.563885)  (0.385458,-1.563885)
    x³ + (3,0)·x + (3,0) = 0  (-0.817732,0.000000)  (0.408866,1.871233)  (0.408866,-1.871233)
    x³ + (4,0)·x + (4,0) = 0  (-0.847708,0.000000)  (0.423854,2.130483)  (0.423854,-2.130483)
    x³ + (5,0)·x + (5,0) = 0  (-0.868830,0.000000)  (0.434415,2.359269)  (0.434415,-2.359269)
    x³ + (6,0)·x + (6,0) = 0  (-0.884622,0.000000)  (0.442311,2.566499)  (0.442311,-2.566499)
    x³ + (7,0)·x + (7,0) = 0  (-0.896922,0.000000)  (0.448461,2.757418)  (0.448461,-2.757418)
    x³ + (8,0)·x + (8,0) = 0  (-0.906795,0.000000)  (0.453398,2.935423)  (0.453398,-2.935423)

### See also

pow(std::valarray) | applies the function std::pow to two valarrays or a valarray and a value (function template)
sqrtsqrtfsqrtl (C++11)(C++11) | computes square root (\(\small{\sqrt{x}}\)√x) (function)
sqrt(std::complex) | complex square root in the range of the right half-plane (function template)

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